Cemented carbide body with increased wear resistance

ABSTRACT

As there is disclosed a cemented carbide body comprising WC with an average grain size of &lt;10 μm in a binder phase. In the cemented carbide body the WC grains can be classified in at least two groups in which a group of smaller grains has a maximum grain size a max  and a group of larger grains has a minimum grain size b min  and each group contains at least 10 % of the total amount of WC grains. According to the invention b min −a max &gt;0.5 μm and the difference in grain size within each group is &gt;1 μm.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,210,632 B1,which is a national stage application of PCT/SE97/01242 filed on Jul. 8,1997, and which claims the benefit of priority to Swedish ApplicationNo. 9602812 - 1 filed Jul. 19, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to coated cemented carbide bodiesparticularly useful in tools for turning, milling and drilling of steelsand stainless steels.

Cemented carbide bodies are manufactured according to powdermetallurgical methods including milling, pressing and sintering. Themilling operation is an intensive mechanical milling in mills ofdifferent sizes and with the aid of milling bodies. The milling time isof the order of several hours up to days. Such processing is believed tobe necessary in order to obtain a uniform distribution of the binderphase in the milled mixture, but it results in a wide WC grain sizedistribution.

In U.S. Pat. Nos. 5,505,902 and 5,529,804 methods of making cementedcarbide are disclosed according to which the milling is essentiallyexcluded. Instead, in order to obtain a uniform distribution of thebinder phase in the powder mixture, the hard constituent grains areprecoated with the binder phase, the mixture is further wet mixed withpressing agent dried, pressed and sintered. In the first mentionedpatent the coating is made by a SOL-GEL method and in the second, apolyol is used.

EP-A-665 308 discloses a coated cutting insert with a bimodaldistribution of WC grain size with WC grains in two groups 0.1-1 μm and3-10 μm. The insert according to this application is produced withconventional milling technique resulting in a broadening of the WC grainsize distribution.

OBJECT AND SUMMARY OF THE INVENTION

It is an aspect of this invention to provide a method of making acemented carbide body comprising wet mixing without milling of at leasttwo different WC-powders with deagglomerated powders of other carbides,binder metal and pressing agent such that the WC-powders are coated withthe binder phase, said WC-grains being carefully deagglomerated beforeand after being coated with binder metal, the grains of the WC-powderbeing classified in at least two groups in which a group of smallergrains has a maximum grain size a_(max) and a group of a larger grainshas a minimum grain size b_(min), each group containing at least 10% ofthe total amount of WC grains wherein b_(min)−a_(max)>0.5 mm, thevariation in grain size within each group being >1 μm, drying saidmixture, pressing to a desired shape and sintering said pressed bodies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

It has now surprisingly been found that a further improvement of theproperties of a cemented carbide according to EP-A-665 308 can beobtained it such a material is made using the technique disclosed in theabove mentioned U.S. Pat. No. 5,505,902 or 5,529,804.

The present invention relates generally to a cemented carbide bodycomprising WC with an average grain size of <10 μm in a binder phase.The WC grains are classified in at least two groups in which a group ofsmaller grains has a maximum grain size a_(max) and a group of largergrains has a minimum grain size b_(min). Each group contains at least10% of the total amount of WC grains. The cemented carbide bodyaccording to the invention is characterized in that b_(min)−a_(max)>0.5μm and that the variation in grain size within each group is >1 μm.

More particularly, the invention relates to a coated cutting insert witha bimodal distribution of the WC grains particularly useful formachining of steels and stainless steels comprising WC and 4-20 wt-% Co,preferably 5-12.5 wt-% Co and 0-30 wt-% cubic carbide, preferably 0-15wt-% cubic carbide, most preferably 0-10 wt-% cubic carbide such as TiC,TaC, NbC or mixtures thereof. The WC grains have a narrow bimodal grainsize distribution with grain sizes in the ranges 0-1.5 μm and 2.5-6.0 μmrespectively, and with a weight ratio of fine WC particles (0-1.5 μm) tocoarse WC particles (2.5-6.0 μm) in the range of 0.25-4.0, preferably0.5-2.0.

The amount of W dissolved in the binder phase is controlled byadjustment of the carbon content by small additions of carbon black orpure tungsten powder. The W-content in the binder phase can be expressedas the “CW-ratio” defined asCW-ratio=M_(s)/(wt %Co*0.0161)where M_(s) is the measured saturation magnetization of the sinteredcemented carbide body in kA/m hAm² /kg and wt % Co is the weightpercentage of Co in the cemented carbide. The CW-value in insertsaccording to the invention shall be 0.82-1.0, preferably 0.86-0.96.

The sintered inserts according to the invention are used coated oruncoated, preferably coated by MTCVD, conventional CVD or PVD, with orwithout Al₂O₃. In particular, multilayer coatings comprisingTiC_(x)N_(v)O_(z) with columnar grains followed by a layer of α-Al₂O₃,κ-Al₂O₃ or a mixture of α- and κ-Al₂O₃, have shown good results. Inanother preferred embodiment, the coating described above is completedwith a TiN-layer which can be brushed or used without brushing.

According to the method of the present invention, a cemented carbidebody is made comprising wet mixing without milling of at least twodifferent WC-powders with deagglomerated powders of other carbides,generally TiC, TaC and/or NbC, binder metal and pressing agent, driedpreferably by spray drying, pressed to inserts and sintered. The grainsof the WC-powder are classified in at least two groups in which a groupof smaller grains has a maximum grain size a_(max) and a group of largergrains has a minimum grain size b_(min) each group containing at least10% of the total amount of WC grains wherein b_(min)−a_(max)>0.5 μm andthe variation in grain size within each group is >1 μm. Preferably,prior to mixing, the WC grains are carefully deagglomerated before andafter being coated with binder metal.

Particularly according to the method of the present invention,WC-powders with two narrow grain size distributions of 0-1.5 μm and2.5-6.0 μm respectively and a weight ratio of fine WC particles (0-1.5μm) to coarse WC particles (2.5-6.0 μm) in the range of 0.25-4.0,preferably 0.5-2.0 are wet mixed without milling with other carbidesgenerally TiC, TaC and/or NbC, binder metal and pressing agent, driedpreferably by spray drying, pressed to inserts and sintered.

It is essential according to the invention that the mixing takes placewithout milling, i.e., there should be no change in grain size or grainsize distribution as a result of the mixing.

In a preferred embodiment, the hard constituents, at least those withnarrow grain size distributions, are after careful deagglomerationcoated with binder metal using methods disclosed in U.S. Pat. No.5,505,902 or 5,529,804. In such case, the cemented carbide powderaccording to the invention is preferably of Co-coated WC+Co-binder, withor without additions of the cubic carbides such as TiC, TaC, NbC,(Ti,W)C, (Ta,Nb)C, (Ti,Ta,Nb)C, (W,Ta,Nb)C, and (W,Ti,Ta,Nb)C coated oruncoated, preferably uncoated, possibly with further additions ofCo-powder in order to obtain the desired final composition.

The invention is additionally illustrated in connection with thefollowing Examples which are to be considered as illustrative of thepresent invention. It should be understood however, that the inventionis not limited to the specific details of the Examples.

EXAMPLE 1

A. Cemented carbide tool inserts of the type SEMN 1204 AZ, an insert formilling, with the composition in addition to WC of 8.4 wt % Co, 1.13 wt% TaC and 0.38 wt % NbC were produced according to the invention. Cobaltcoated WC, WC-6 wt-% Co, prepared in accordance with U.S. Pat. No.5,505,902 was carefully deagglomerated in a laboratory jetmillequipment, mixed with additional amounts of Co and deagglomerateduncoated (Ta,Nb)C and TaC powders to obtain the desired materialcomposition. The coated WC-particles consisted of 50 wt % with anaverage grain size of 3.5 μm and 50 wt % with 1.2 μm average grain size,giving a bimodal grain size distribution. The mixing was carried out inan ethanol and water solution (0.25 l fluid per kg cemented carbidepowder) for 2 hours in a laboratory mixer and the batch size was 10 kg.Furthermore, 2 weight-% lubricant, was added to the slurry. The carboncontent was adjusted with carbon black to a binder phase alloyed with Wcorresponding to a CW-ratio of 0.89. After spray drying, the insertswere pressed and sintered according to standard practise and densestructures with no porosity were obtained.

Before coating a negative chamfer with an angle of 20 degrees was groundaround the whole insert.

The inserts were coated with a 0.5 μm equiaxed TiCN-layer (with a highnitrogen content corresponding to an estimated C/N-ratio of 0.05)followed by a 4 μm thick TiCN-layer with columnar grains by using theMTCVD-technique (temperature 885-850° C. and CH₃CN as the carbon andnitrogen sources). In subsequent steps during the same coating cycle, a1.0 μm thick layer of Al₂O₃ was deposited using a temperature 970° C.and a concentration of H₂S dopant of 0.4% as disclosed in EP-A-523 021.A thin (0.3 μm) layer of TiN was deposited on top according to knownCVD-technique. XRD-measurement showed that the Al₂O₃-layer consisted of100% κ-phase.

The coated inserts were brushed by a nylon straw brush containing SiCgrains. Examination of the brushed inserts in a light microscope showedthat the thin TiN-layer had been brushed away only along the cuttingedge leaving there a smooth Al₂O₃-layer surface.

Coating thickness measurements on cross sectioned brush samples showedno reduction of the coating along the edge line except for the outerTiN-layer that was removed.

B. Cemented carbide tool inserts of the type SEMN 1204 AZ, an insert formilling, with the composition 9.1 wt % Co, 1.23 wt % TaC and 0.30 wt %NbC and the rest WC with unimodal distribution and an average grain sizeof 1.2 μm were produced in the following way. Cobalt coated WC, WC-6weight-% Co, prepared in accordance with U.S. Pat. No. 5,505,902 wascarefully deagglomerated in a laboratory jetmill equipment, mixed withadditional amounts of Co and deagglomerated uncoated (Ta,Nb)C and TaCpowders to obtain the desired material composition. The mixing wascarried out in an ethanol and water solution (0.25 l fluid per kgcemented carbide powder) for 2 hours in a laboratory mixer and the batchsize was 10 kg. Furthermore, 2 weight-% lubricant, was added to theslurry. The carbon content was adjusted with carbon black to a binderphase highly alloyed with W corresponding to a CW-ratio of 0.89. Afterspray drying, the inserts were pressed and sintered according tostandard practise and dense structures with no porosity were obtained.

Before coating a negative chamfer with an angle of 20 degrees was groundaround the whole of each insert.

The inserts were coated in the same coating batch as the inserts Aabove.

The coated inserts were brushed by a nylon straw brush containing SiCgrains. Examination of the brushbed inserts in a light microscope showedthat the thin TiN-layer had been brushed away only along the cuttingedge leaving there a smooth Al₂O₃-layer surface.

Coating thickness measurements on cross sectioned brushed samples showedno reduction of the coating along the edge line except for the outerTiN-layer that was removed.

C. Cemented carbide tool inserts of the type SEMN 1204 AZ with the samechemical composition, average grain size of WC, CW-ratio, chamfering,CVD-coating and brushing respectively as the insert B above but producedfrom powder manufactured with conventional ball milling techniques wereused as reference for comparison with the test specimens according toabove.

Inserts from A, B and C were compared in a wet milling test in a ratherhighly alloyed steel (HB=310). Two parallel bars each of a thickness of35 mm were centrally positioned relative the cutter body (diameter 100mm ), and the bars were placed with an air gap of 10 mm between them.

The cutting data were:

-   -   Speed=150 m/min    -   Feed=0.40 mm/rev    -   Cutting depth 2 mm, single tooth milling with coolant.

Evaluated tool life expressed as cutting length of variant A accordingto the invention was 8200 mm and for variant B 6900 mm and finally forthe standard variant C only 6100 mm. In this test the insert accordingto the invention with a bimodal WC grain size distribution, variant A,obtained the best result.

EXAMPLE 2

A. Inserts from the same batch as insert A in Example 1 above and

B. Inserts from the same batch as insert B in Example 1 above and

C. Inserts from the same batch as insert C in Example 1 above

were compared in a wet milling test in a low alloyed steel (SS 1650,HB=180). Two parallel bars each of a thickness 30 mm were centrallypositioned relative the cutter body (diameter 100 mm). The bars wereplaced with an air gap of 10 mm between them.

The cutting data were:

-   -   Speed=285 m/min    -   Feed=0.38 mm/rev    -   Cutting depth 2 mm, single tooth milling with coolant.

Evaluated tool life expressed as cutting length of variant A accordingto the invention was 4800 mm and for variant B, 4200 mm and finally forthe standard variant C only 3600 mm. In this test the insert accordingto the invention with a bimodal WC grain size distribution, variant A,performed best.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention is intended to be protected herein, however, is not to beconstrued as limited to the particular forms disclosed, since these areto be regarded as illustrative rather than restrictive. Variations andchanges may be made by those skilled in the art without departing fromthe spirit of the invention.

1. A method of making a cemented carbide body comprising wet mixingwithout milling of at least two different WC-powders with deagglomeratedpowders of other carbides and a binder metal such that the WC-powersWC-powders are coated with the binder phase, said WC-grains beingdeagglomerated before and after being coated with binder metal, thegrains of the WC-powder being classified in at least two groups in whicha group of smaller grains has a maximum grain size a_(max) and a groupof larger grains has a minimum grain size b_(min), each group containingat least 10% of the total amount of WC grains whereinb_(min)−a_(max) >0.5 mm μm, the variation in grain size within eachgroup being >1 μm, drying said mixture, pressing to a desired shape andsintering said pressed bodies.
 2. The method of claim 1 wherein saidother carbides comprise one or more of TiC, TaC and NbC.
 3. The methodof claim 1 wherein said sintered bodies are coated with an Al₂O₃ layer.4. The method of claim 1 wherein said two groups of WC-powder have grainsize distributions of 0-1.5 μm and 2.5-6.0 μm, respectively.
 5. Themethod of claim 4 wherein the weight ratio of particles with a grainsize distribution of 0-1.5 μm to 2.5-6.0 μm is from 0.25 to 4.0.
 6. Themethod of claim 5 wherein said weight ratio is from 0.5-2.0.
 7. Themethod of claim 1 wherein said sintered bodies have a CW-ratio of0.82-1.0.
 8. The method of claim 7 wherein said CW-ratio is 0.86-0.96.9. The method of claim 3 wherein said sintered bodies have a CW-ratio of0.82-1.0.
 10. The method of claim 9 wherein said CW-ratio is 0.86-0.96.11. The method of claim 4 wherein said sintered bodies have a CW-ratioof 0.82-1.0.
 12. The method of claim 11 wherein said CW-ratio is0.86-0.96.
 13. The method of claim 5 wherein said sintered bodies have aCW-ratio of 0.82-1.0.
 14. The method of claim 13 wherein said CW-ratiois 0.86-0.96.